Regard a vessel as an aid to beauty (8)
I believe the answer is:
eyeliner
'an aid to beauty' is the definition.
I can't tell whether this definition defines the answer.
'regard a vessel' is the wordplay.
'regard' becomes 'eye' (I've seen this before).
'a vessel' becomes 'liner' (liner is a kind of vessel).
'eye'+'liner'='EYELINER'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for eyeliner that I've seen before include "kind of cosmetic" , "instrument for enhancing lamps" , "Type of make-up" , "it draws attention to the viewer" , "Facial cosmetic" .)